The present invention relates to a selection diversity using a decision feedback equalizer which is used in digital mobile telecommunication systems such as a digital auto-mobile telephone system, a digital portable telephone system and a digital cordless telephone system, and digital radio telecommunication systems such as a digital satellite communication system and a digital mobile satellite communication system.
To meet with the recent demand for digitization of automobile telephone, portable telephone and similar land mobile telecommunication systems, a variety of digital radio techniques are now being studied, developed and put to practical use. While at present a medium speed system of a 42 kbps information transmission rate is being adopted as a next-generation automobile telephone system, it is being desired to develop a wide band digital mobile telecommunication system whose transmission rate is as high as hundreds to several megabytes per second so as to meet with a future demand for higher transmission efficiency.
Incidentally, such a broad band digital mobile telecommunication system seriously suffers deterioration of bit error rate performance which is caused by the degradation of the receiving signal level owing to Rayleigh fading inherent in the land mobile telecommunication system and deterioration of the bit error rate which is caused by intersymbol interference owing to frequency selective fading. As a solution to the former problem presented by the degradation of the receiving signal level owing to the Rayleigh fading, there is now being studied and developed a space diversity receiving system which improves the receiving performance by receiving signals with a plurality of spaced-apart antennas and selecting or combining the received signals. As a solution to the latter problem by the intersymbol interference owing to the frequency selective fading, there is now being studied and developed a decision feedback equalizer which feeds the result of decision at its output back to a feedback tap of a transversal filter which estimates and implements an inverse response of a time response in the frequency selective fading communication channel. Up to now, a selection diversity system using a decision feedback equalizer shown in FIGS. 3 and 4 have been proposed as demodulating systems which combines the above-mentioned two transmission techniques.
FIG. 3 shows a two-branch, selection diversity system using a decision feedback equalizer which selects a branch by comparing receiving levels with each other. Independent receiving circuits I(12), II(12) and equalizers I(14), II(24) are provided in association with two antennas 11 and 12 of branches I and II, respectively. An output R.sub.1 (104) from a received level detector I(103) which detects the level of a received signal in the branch I and an output R.sub.2 (204) from a received level detector II(203) which detects the level of a received signal in the branch II are compared with each other by a received signal level comparator 54, and a decision value of the equalizer output of the branch of the higher received signal level is selected by a branch selector 33, from which it is output as a decision data sequence 34. In the frequency selective fading communication channel, however, the branch of the higher received signal level is not always high in equalization accuracy but the branch of the lower received signal level may sometimes be excellent in equalization accuracy. This leads to a problem that the equalizer using the diversity receiving system degrades the receiving performance more than does an ordinary equalizer which does not utilize the diversity receiving system. One possible solution to this problem is proposed in Japanese Pat. Pub. No. 111542/92 entitled "Diversity Method." FIG. 4 shows the construction of a receiving apparatus of this "diversity method" and FIG. 5 the construction of a decision feedback type equalizer for use in the receiver.
In the diversity receiver depicted in FIG. 4, the independent receiving circuits I (12), II (13) and equalizers I (14), II (24) are provided in association with the two antenna branches I (11), II (21), respectively. An output E.sub.1 (17) from an estimation error average value detector I (16) which detects the average value of estimation errors e.sub.1 (t) (15) of the equalizer I (14) over a plurality of symbols in the branch I and an output E.sub.2 (27) from an estimation error average value detector II (26) which detects the average value of estimation errors e.sub.2 (t) (25) of the equalizer II over a plurality of symbols in the branch II are compared with each other by an estimation error comparator 31, and the equalizer output of the branch decided to be smaller in estimation error average value than in the other branch is selected by the branch selector 33, from which it is output as the decision data sequence 34.
In FIG. 5 the decision feedback equalizer comprises: a feed-forward filter 37; a feedback filter 43; an adder 40 for adding their outputs 38 and 44; a decision circuit 41 for deciding the adder output 40; an estimation error detector 45 for detecting a difference between the decision circuit output 42 and the adder output 40; and a tap gain coefficient update circuit 51 for calculating the tap gain coefficients by the use of an estimation error 57 and for updating the tap gain coefficients accordingly. The estimation error 57 is used to calculate tap gain coefficients and updated tap gain coefficients 52 and 53 are used to update the taps of the feed-forward filter 43 and the feedback filter 37, whereby it is possible to accurately equalize intersymbol interference in accordance with a change of the propagation path. By selecting a branch through utilization of the average value of estimation errors and combining decision values of the equalizer output of the selected branch as a data sequence as in the above-mentioned "diversity system," it is possible to solve the problem of the selection diversity system using a decision feedback equalizer which selects the branch according to the receiving signal level as referred to above; hence, it is possible to improve the receiving performance by virtue of the diversity effect.
In FIG. 6 there are shown an equalization accuracy performance of the equalizer of each branch in the conventional selection diversity system using a decision feedback equalizer and a phenomenon of a selection error in the branch. In the interests of brevity, the number of branches shown is two and it is assumed here that communications take place in the form of a burst signal on the supposition of the use of a TDMA (Time Division Multiple Access) system. Usually, a TDMA frame is composed of a unique word part (corresponding to a training sequence in the equalizer) made up of a known symbol sequence and an information signal part made up of an unknown symbol sequence.
In FIG. 6, the equalizers of the branches I and Ii normally operate in sections d11 and d21, but in sections d12 and d22 a step out is caused by a decrease in the receiving signal level owing to fading and the equalization algorithm becomes divergent. It will be seen from FIG. 6 that in the case of a method of merely comparing with each other the estimation errors of the two branches and selecting one of them accordingly, the estimation error of the branch I happens to be smaller than the estimation error of the branch II in a section c12 although the equalizer of the branch II is normal, with the result that a selection error is induced by the application of the diversity system. Furthermore, in a section c13 (d22) the both branches are out of synchronism and hence are in a random error state and the equalization algorithm will not be pulled in again in one frame.
In the selection diversity system using a decision feedback equalizer which selects one of branches on the basis of the result of comparison of their average estimation errors, the decision feedback equalizers of the respective branches are independent of one another and the result of decision of the equalized output from the decision feedback equalizer in a certain branch does not exert any influence on the decision feedback equalizers of other branches. That is, the performance of the decision feedback type equalizer in each branch is the performance itself of an ordinary decision feedback type equalizer which is obtainable in the case of reception of one branch, and improvement of the receiving performance by the application of the diversity receiving system is no more than the effect produced by merely avoiding a communication channel in a poor condition of propagation. Hence, it cannot be said that the conventional selection diversity system using a decision feedback equalizer takes full advantage of the merits brought about by the application of the diversity receiving system to the decision feedback equalizer.